1. Field of the Invention
This invention relates to a data transmission method and, more particularly, to a data transmission method capable of shortening data transmission time.
2. Description of the Related Art
In a system equipped with a machine such as a machine tool or robot and a numerical control unit (referred to as an "NC unit"), desired machining control or robot service control is carried out while the machine and NC unit exchange signals with each other. As an example, if a miscellaneous function instruction (M-function instruction), tool change instruction (T-function instruction) and spindle rotational velocity instruction (S-function instruction) to be sent from an NC tape or the like to the machine side are read, the NC unit sends these instructions to the machine, and the machine receives these instructions, executes a machining operation designated by the instructions and, with the completion of the machining operation, sends an operation completion signal back to the NC unit. Sensors and various limit switches such as a limit switch for over-travel detection and a limit switch for deceleration are attached to the machine, and the states of these limit switches and sensors are sent to the NC unit, in response to which the NC unit executes predetermined processing.
In a case where parallel data (assumed here to be m-bit data) from the NC unit to the machine are transmitted on m-number of transmission lines, there are occasions where not all bits of the parallel data arrive at the machine simultaneously. There are various reasons for this, such as a disparity in the delay exhibited by receivers provided in the machine. Accordingly, as shown in FIGS. 1 and 2, an arrangement is adopted in which an NC unit 101 sends parallel data DT to a machine 102 via m-number of transmission lines Lm and then, at expiration of a predetermined period of time t.sub.1, sends a data transmission signal DS to the machine via a transmission line Ls, with the machine 102 responding to the receipt of the data transmission signal Ds by reading the parallel data DT. The predetermined period of time t.sub.1 is that amount of time needed for all bits of the parallel data DT to arrive in unison at the machine. After reading the parallel data DT, the machine 102 sends a data reception signal DR to the NC unit 101 via a transmission line Lr, and the NC unit 101 sets the data transmission signal DS to a low level upon receiving the data reception signal DR. The machine 102 sets the data reception signal DR to the low level in response to the low level to which the data transmssion signal DS has been set. The foregoing processing ends the transmission cycle for one item of parallel data and places the NC unit 101 in a state capable of transmitting the next item of parallel data. Thereafter, items of parallel data are transmitted successively through identical cycles of operation.
Thus, according to the conventional transmission method, one item of parallel data is transmitted through a cycle comprising:
(1) transmitting the parallel data DT; PA0 (2) sending the data transmission signal DS; PA0 (3) reading the parallel data and sending back the data reception signal DR; PA0 (4) setting the data transmission signal DS to the low level; and PA0 (5) setting the data reception signal DR to the low level. Consequently, though the data can be transmitted in a reliable manner, a long period of time is required for the transmission. This is the drawback of the conventional method. PA0 (1) transmitting parallel data DT; PA0 (2) sending a data transmission signal of a high level (e.g., logical "1"); and PA0 (3) reading the parallel data in response to receipt of the high-level data transmission signal and sending back a high-level data reception signal; a subsequent transmission cycle is completed by the three steps of: PA0 (1)' transmitting parallel data DT; PA0 (2)' sending a data transmission signal of a low level (e.g., logical "0"); and PA0 (3)' reading the parallel data in response to receipt of the low-level data transmission signal and sending back a low-level data reception signal; and data transmission is subsequently carried out by repeating the transmission cycles of the first and second parallel data each time a parallel data transfer takes place, thereby enabling the data transmission time to be shortened.